1. Field of the Invention
The present invention relates to a wafer polishing method and apparatus in semiconductor fabrication equipment, and more particularly to a wafer polishing apparatus and method capable of sufficiently controlling the polishing quantity of a wafer serving as a semiconductor substrate and continuously polishing the wafer at a stable polishing quantity.
2. Description of the Related Art
Various micro-fabrication techniques have been researched and developed in order to improve integration degree and functions of an LSI. Attention has been paid to a chemical mechanical polishing (hereinafter referred to as “CMP”) technique which is one of the micro-fabrication techniques. The CMP is a polishing technique with a combination of a chemical action between an abrasive (slurry) and an object to be polished (wafer), and a mechanical action by abrasive grains in the slurry. This is technique useful in flattening an insulating film, forming a metallic plug, and forming an embedded wiring in a semiconductor fabrication process.
A wafer polishing apparatus for performing the CMP comprises a polishing surface plate (platen) to which an abrasive cloth (abrasive pad) is attached, a head for holding the back surface of a wafer in order to press the front surface of a wafer to be polished against the abrasive pad, a driving mechanism for relatively rotating the platen and wafer, and a supply mechanism for supplying slurry between the abrasive pad and wafer.
The abrasive pad is typically made of elastic polyurethane or the like. Because abrasion or loading occurs on the surface of the abrasive pad due to polishing, the abrasive pad is damaged and thereby, the polishing efficiency is deteriorated. For this reason, a conditioning mechanism is provided which presses a conditioner to which diamond is attached by electrodeposition against the surface of the abrasive pad performs honing at a proper timing before, after, or during a polishing step.
In a conventional conditioning method disclosed in Japanese Patent Laid-Open No. 2000-117615, fluctuations in polishing quantity are checked by polishing a dummy wafer, measuring film thicknesses before and after polishing the dummy wafer by a film thickness measuring instrument, and calculating a polishing quantity in accordance with the differences between the film thicknesses, when the predetermined number of wafers have been polished. When the calculated polishing quantity is out of a set range, conditioning is executed. However, the polishing quantity does not fall within the set range yet, conditioning is executed again by changing conditioning terms. Thus, polishing and conditioning of the dummy wafer are repeated until the polishing quantity falls within the set range through trial and error. In the case of this method, however, because it is necessary to regularly polish and examine the dummy wafer whenever the polishing is executed in predetermined times, lots of time and labor are necessary and this causes the wafer fabrication cost to increase.
Therefore, Japanese Patent Laid-Open No. 2000-202758 discloses a control method of the surface state of an abrasive pad using a mechanism in which a conditioner and the rotating shaft of a motor for rotating the conditioner are connected by a balanced spring. This is a method for detecting a deflection value and making the deflection value fall within a set range, based on the deflection of a rotating balanced spring due to a friction force when honing an abrasive pad. In this method, the honing force of an abrasive pad by a conditioner is obtained from a detected deflection value and quantitatively controlled. This method is particularly effective for initial conditioning when using a brand-new abrasive pad.
However, as described above, the method for detecting the friction force between a conditioner and an abrasive pad in accordance with the deflection value of a balanced spring has a problem that the sensitivity is too low. This is because a structure is used in which a balanced spring for being detected its deflection value is not easily deflected since it rotates together with a detection section and moreover, a pressure is applied. To efficiently hone an abrasive pad, it is preferable to increase the rotational speed of a conditioner and increase a pressure. However, when increasing the rotational speed of the conditioner, the balanced spring is more hardly deflected and the sensitivity is further deteriorated. To compensate the above phenomena, a soft balanced spring may be used. However, even in this case, the sensitivity is also deteriorated when the pressure of the conditioner increases. Moreover, when using the soft balanced spring, the balanced spring is more easily deformed due to the friction force when honing an abrasive pad and the torsion of the balanced spring is increased due to the rotation of the conditioner. Therefore, a problem occurs that not only the deflection of the balanced spring but also the torsion of the spring are detected and the surface state of the abrasive pad cannot be accurately detected. It is possible to electrically simply amplify a detection value in order to detect a very small deflection value of a balanced spring. In this case, however, because noises due to the rotation of the conditioner and abrasive pad increase and thereby, the detection accuracy of the surface state of the abrasive pad is deteriorated.
Thus, the above conventional method is effective even if the rotation of a conditioner increases and a large pressure is applied when a deflection value difference is large, for example, surface states (honed status) are greatly changed like when a present abrasive pad is changed to a brand-new abrasive pad. However, when the fluctuations in surface state are small, for example, while wafer polishing is continuously performed, it is difficult to detect a deflection value difference because it is too small and accurately confirm the surface state of an abrasive pad.